Like Tuner571, I think it's a focus problem. At 1/4000", it's not likely to be camera shake or motion blur
Looking at the second photo, it seems to me that the name on the side of the second trailer is in better focus than the name on the first trailer. It could be less blur from the rain. Maybe my old eyes are deceiving me. Are you able to do some pixel peeping to check? If the focus point is closer to the rear of the second trailer, it would be somewhere in the middle of the cloud of raindrops at the centre of the image.
One normally thinks of very narrow depth of field at f 1.4, but we're normally shooting things at much closer distance with that aperture. What would you estimate the distance to the truck? 30 metres? At that distance the depth of field would be 23 metres, so it's not a depth of field problem.
I've been trying for road train pictures for years and haven't got a good one yet. We don't get them in Melbourne. Lots of B-doubles and the occasional triple. My opportunities are limited to the Christmas holidays when we drive up to QLD. I missed several opportunities at triple-trailer road trains on the Gore Hwy between Toowoomba and Goondiwindi on the last trip. One day I'll have the time to stop for an hour or so. If you ever discover the secret, please let me know.
Richard.